A vehicle-mounted radar for detecting a distance to a target and a relative speed of the target is classified into various types according to modulation of a radar used Therefore. For example, “Current Status and Trends of mm-Wave Automotive Radar” in Journal of Institute of Electronics, Information and Communication Engineers, October 1996, (pp. 977-981) discloses a dual-frequency CW (Continuous Wave) system, an FMCW (Frequency Modulated Continuous Wave) system and a pulse system as such systems. The radar of the dual-frequency CW system alternately transmits two kinds of continuous waves having different frequencies, mixes the echoes of the transmitted waves, and detects a relative speed of the target and a distance to the target according to a frequency and a phase of the obtained signal. The radar of the FMCW system transmits a continuous wave having frequency modulation done by a triangular wave and also mixes the transmitted wave and the echo from the target, and detects a distance to the target and a relative speed of the target based on the frequency of a beat signal obtained. The radar of the pulse system transmits a pulse wave, counts a lapse of time until its echo is received, and detects a distance to the target based on the counted value.
It is also possible to detect a deviation angle (hereinafter called as an azimuth angle) of a target direction with respect to the reference direction by the radars of such systems. As systems □herefore, Sensor Actuator/Week '99 General Symposium: Automobile and Sensor Technology, “mm-Wave Radar-Assisted Vehicle-to-Vehicle Distance Warning Device” discloses a mechanical scanning system, a beam switching system and a monopulse system. These three systems are summarized below.
(1) Mechanical Scanning System
As shown in FIG. 1, the radar of the mechanical scanning system scans a horizontal plane with a main beam A from a transmitting/receiving antenna 11 by oscillating the transmitting/receiving antenna 11 about an axis O by a servomotor or the like. This mechanical scanning-type radar needs to expand a turning angle Ø of the transmitting/receiving antenna 11 in order to expand a scanning range θ by the main beam A from the transmitting/receiving antenna 11. To do so, it is necessary to have a larger depth in a casing 10, in which the transmitting/receiving antenna 11 is accommodated, so not to disturb the transmitting/receiving antenna 11 from oscillating. For example, to change the scanning range θ by the main beam A from the transmitting/receiving antenna 11 from an angle θ1 of a level of a horizontal beam width to an angle θ2 larger than that as shown in FIG. 2, it is necessary to change a depth D of the housing to at least a value D2 larger than a value D1 of a level of the thickness of the transmitting/receiving antenna 11. Therefore, the radar itself comes to have a large size.
(2) Beam Switching System
As shown in FIG. 3, the radar of the beam switching system scans a horizontal plane by the main beam from an antenna 30 by sequentially switching transmitter-receivers 31S1, 31S2, . . . , 31Sn on the antenna 30 which transmits beams A1, A2, . . . , An having different directions from one another. This beam switching type radar needs to increase the number of the transmitter-receivers 31S1, 31S2, . . . , 31Sn on the antenna 30 to expand the scanning range θ by the main beam from the antenna 30. To do so, it is necessary to secure a space for arrangement of add-on receivers by increasing a horizontal width of the antenna 30. For example, as shown in FIG. 4, it is necessary to change a width W of the antenna 30 from the present value W3 to a value W4 larger than that in order to change the scanning range θ by the main beam from the antenna 30 from the present angle θ3 to an angle θ4 which is larger than that. Therefore, the radar itself comes to have a large size.
(3) Monopulse System
The radar of the monopulse system receives echoes from a target by two antennas which are disposed side to side to detect an azimuth angle of the target based on a phase difference of the waves received by the two antennas.
The above-described radars of the individual systems are started to be in practical use as a vehicle-mounted radar predicated on use on freeways as described in Nikkei Electronics, Feb. 22, 1999 (pp. 47-53).